Physics · Alternating Current · NEET
Pick one quantity to point along the positive x-axis (0 degrees) and measure every other angle from it. In a single element or a series circuit, the current is the same everywhere, so we usually draw the current phasor as the reference along the x-axis. Then place each voltage arrow at its correct angle: for a resistor the voltage is on the same line as current (0 degrees), for an inductor the voltage is 90 degrees ahead (turned anticlockwise), and for a capacitor the voltage is 90 degrees behind (turned clockwise).
By the standard NCERT definition the phasor length equals the peak (maximum) value, because the vertical component of the rotating arrow traces v = v_m sin(omega t). So an arrow of length v_m gives instantaneous voltage v = v_m sin(omega t). You may also draw phasors using rms values (V_rms = V_m / 1.414) as long as you keep all arrows in the same units, because the angles between them stay the same. For NEET, state which one you use; peak is the default.
Phasors rotate anticlockwise about the origin with angular speed omega (the same omega as the source). At every instant the whole set of arrows turns together like the spokes of a wheel, so the angles between them never change. That is why a phasor diagram is just a frozen snapshot: you draw the arrows at one convenient instant, and the fixed angles between them carry all the information you need.
The instantaneous AC value is a sine function: v = v_m sin(omega t). When an arrow of length v_m is at angle omega t from the x-axis, its vertical (y-axis) projection is exactly v_m sin(omega t). So the vertical component of the rotating arrow at any instant equals the real instantaneous value of the voltage or current. The arrow itself is just a bookkeeping tool; only its vertical shadow is the physical AC quantity.
Use the parallelogram or head-to-tail rule, just like vectors. If two arrows are perpendicular, the resultant length is the Pythagoras combination: R = sqrt(A^2 + B^2). For example, in a series RL circuit V_R (along current) and V_L (90 degrees ahead) give source voltage V = sqrt(V_R^2 + V_L^2), and the phase angle is tan(phi) = V_L / V_R. This is the whole trick behind LCR impedance: add the perpendicular voltage arrows with Pythagoras.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A phasor is an arrow that spins about the origin at the AC angular speed omega. Its length is the peak value of the quantity, and its vertical shadow at any moment gives the real instantaneous AC value. Voltage and current are shown as phasors only to make adding them easy; they are not true vectors.
Either is fine as long as all arrows use the same unit, because the angles do not change. NCERT's definition uses peak values (v_m, i_m). For quick NEET problems many students use rms so the numbers match the meter readings; just be consistent and state your choice.
In a series AC circuit the same current flows through every element, so it is the one quantity common to all of them. Drawing current along the x-axis lets you place each element's voltage at its correct lead or lag angle, which makes the diagram easy to add up.
The phase angle phi is the angle between the source voltage phasor and the current phasor. In a series RL or RC circuit it comes from tan(phi) = (net reactive voltage) / (resistive voltage), for example tan(phi) = V_L / V_R. A positive phi means voltage leads current; a negative phi means voltage lags current.
They are closely related. Divide every voltage phasor by the common current and the voltage triangle becomes the impedance triangle: R along the current direction, X_L up, X_C down, and Z as the hypotenuse. So an impedance diagram is a phasor diagram scaled by the current.
